Warning: Some spoilers ahead. Though if you've either read the book or seen the movie, you're cool.
Order matters. It's not true what they say: that as between a book and a movie, you inevitably prefer whichever one you read or watch first. But the order you take them in nonetheless profoundly affects the experience. You can try to be objective – claim that each work has to stand or fall on its own merits, other incarnations be damned – but it won't work. You've been tainted.
I liked The Ruins – the movie. It was tight, brutal, ruthlessly effective; along with The Strangers, one of the year's few R-rated breaths of fresh air. Though it hewed pretty closely to genre conventions, it also recombined them to come up with its own interesting take on survival horror. I appreciated the movie's simplicity (the vines are a pure, almost elemental villain); its gruesomeness that never turned into sadism or needless cruelty; its grim, harsh relentlessness. It was a gripping roller coaster of a movie; a fun ride I enjoyed, praised, and pretty much put out of my mind.
Now that I've read the book, I ask myself: Would I still have liked the movie had I gone to the book first? The answer, I think, is no. It's not that I now think I was wrong about the film; to the contrary. But Scott Smith's novel is so extraordinary a genre achievement that the movie – adapted by Smith himself – can, in retrospect, feel only like a hapless abridgement, a wispy simulacrum of the novel's all-encompassing sense of doom and spiraling psychological terror. Taking the two in reverse order would have made the film feel cheap, impotent, lame; The Ruins for Dummies.
As Bryant Frazer has pointed out, there are no actual ruins in the book, as there are in the film; the title refers not to an archeological site but rather to what the characters – initially fresh-faced, well-to-do twenty-somethings on a Cancún getaway – become toward the end of their ordeal. In the novel, that ordeal is harrowing and attenuated, as the characters, the Mayans who keep them quarantined, and the vines, fight a war of attrition that our heroes are sure to lose. Their bodies are wracked by thirst, hunger and injury; their minds by denial, bitterness and guilt. Smith's razor-sharp, uncommonly lucid prose manically shifts perspective, getting us inside each character's head as the six – no, five – no, four -- of them slowly descend into physical and psychological hell. Reading through this is painful, difficult, and unpleasant, but the book is impossible to put down.
The vines become characters – or, better, a character – in the novel, as the extent of their sentience and treachery is gradually rendered clear. If The Ruins is a monster story, it's one in which the monster plots, and schemes, and relishes seeing its prey squirm and suffer. There is the terrifying suggestion (which ultimately becomes the only logical conclusion) that what the characters are facing isn't merely a bunch of mutant plants – hungry, malevolent Venus fly traps – but a thinking, unitary entity; the vines are limbs.
The movie, sadly, doesn't have much time to reflect on either the characters' descent into madness or the nature of what they're up against. This is not (necessarily) a knock. A punchy, 91-minute shocker is just fundamentally a different experience than a 500-page novel with space to explore the nooks and crannies of its premise. And so the movie, by and large, hits the highlights: the shaft, the impromptu leg surgery, the vines' ability to mimic sounds and human voices. The psychological torment aspect of the book is downplayed; the vines are mostly just creepy and menacing rather than existentially terrifying (the truly awful things they do in the last 100 pages of the novel largely don't make it to the screen); the ending is at least fifty times less depressing. It's a smaller work, through and through.
Now, it bears repeating that Scott Smith wrote his own screenplay adaptation (which he also did for Sam Raimi's excellent A Simple Plan). No doubt he knew what he was doing. An experience as draining and exhausting as the book – which most people would read over the course of a couple of days, if not more – would be basically unbearable if packed into two hours. Viewers looking to the horror genre for escapism would hate it, and no studio would touch it. The movie is still plenty disturbing as it stands. But having read the book, the movie seems puny somehow, a cheap imitation. In reality it's probably more like a decent compromise. But when I said I couldn't be objective, I meant it.